How can I tell if stdout is redirected to a channel (and not to a file, character device, terminal, or socket)?

Question

Ideally, this would be scriptable in the shell, but Perl or Python would be fine. The C code may be useful, but probably does not cope with the costs / benefits.

I understand that redirecting to a FIFO (named pipe) can be indistinguishable from a real pipe, and that’s enough for me to really care.

Optimal POSIX solutions are best suited for UNIX / Linux, whatever the option, but at least I need something that works on Darwin (MacOS X).

Before writing my answer - I already know about test -t - this will tell me whether stdout is a terminal (in which case it is definitely not a pipe, but it will not tell me whether stdout was redirected to a file, a nonterminal character device or a UNIX socket -domain, not per channel.

Estimated use case: I have a command that needs to be run inside backquotes in a shell so that it can output commands that set environment variables. I would like the command to interrupt with an error if stdout is not redirected to the channel, since in this case it was definitely not called by eval `mycommand`; .

If there is a special environment variable that the shell will set when running the command in backquotes, which would be useful, but since it is likely to be specific to bash or zsh or something more important, channel detection is more important.

Answer 1

(my comment turned to answer Alex Dupuy's suggestion) Since you said you can use perl, I think you can use its -p file test operator ( perldoc.perl.org/functions/-X.html ); something like perl -e 'exit(-p STDOUT ? 0 : 1);' will tell you if stdout, if there is a channel between them or fifo ( without ).

Fwiw, calling it from the shell exactly why I used perl -e :)

Answer 2

You can do fstat in the file descriptor and check the return structure, for example, st_mode = 0x1000 (S_IFIFO) indicates the Named pipe.

Python example:

 from __future__ import print_function import sys import os print(os.fstat(sys.stdout.fileno()), file=sys.stderr) 

Output to windows:

 C:> python test_fd.py | more nt.stat_result(st_mode=4096, st_ino=0L, st_dev=0, st_nlink=0, st_uid=0, st_gid=0, st_size=0L, st_atime=0L, st_mtime=0L, st_ctime=0L) C:> python test_fd.py > test_fd.txt nt.stat_result(st_mode=33206, st_ino=16888498602769633L, st_dev=0, st_nlink=1, st_uid=0, st_gid=0, st_size=0L, st_atime= 1401119520L, st_mtime=1401119520L, st_ctime=1401119349L) 

Answer 3

The stat command runs on MacOS X and (with some refinement of implementation options) other Linux / Unix variants, so this solution is for me:

 FORMOPT= TYPEFORM= stat() { if [ -z "$FORMOPT" ]; then if /usr/bin/stat --version >/dev/null 2>&1; then FORMOPT=--format TYPEFORM=%F else FORMOPT=-f TYPEFORM=%HT fi fi case $1 in type) FORMARG="$FORMOPT $TYPEFORM" ; shift ;; esac /usr/bin/stat -L $FORMARG " $@ " } exec 9>&1 case `stat type /dev/fd/9` in [Ff]ifo*) echo stdout is a pipe ;; *) echo stdout is not a pipe ;; esac 

Answer 4

In Python:

 import sys print sys.stdout.isatty() 

Examples:

 $ ./a True $ ./a | less False 

Answer 5

There is a Linux-specific solution that does what I want:

 exec 9>&1 case `readlink /dev/fd/9` in pipe:\[*\]) echo stdout is a pipe ;; *) echo stdout is not a pipe ;; esac 

but this is not the answer i'm looking for since i want this to also run on xBSD.

I made this answer from the community wiki, so feel free to improve it, but if you have a solution other than Linux (or a Linux solution using a different technique) create a new answer.